1. Field of the Invention
This invention relates to the controlled regeneration of ion exchangers for the purification and pH adjustment of aqueous solutions using an electrolytic reactor containing a modified ion exchange material for electrolytically splitting the water and generating free radical hydroxyl, free radical hydrogen, regenerant hydroxyl ion and/or regenerant hydrogen ion.
2. Description of the Related Art
As disclosed in Sampson U.S. Pat. Nos. 5,419,816 and 5,609,742, a modified ion exchange material, modified to have both semiconductor junctions and transfer sites, can be used to oxidize or reduce inorganic and organic species in an electrolytic reactor containing the modified ion exchange material. For oxidation, the ion exchange material is a cation exchange material, and the modified cation exchange material is placed in direct contact with at least one anode. When an aqueous solution containing an inorganic or organic species contacts the modified cation exchange material in the presence of a DC current, free radical hydroxyl and hydrogen ion are produced at the semiconductor junctions, and the inorganic or organic species is oxidized. If no species is present to be oxidized, the free radical hydroxyl decomposes to hydroxyl ion.
For reduction, the ion exchange material is an anion exchange material, and the modified anion exchange material is placed in direct contact with at least one cathode. When an aqueous solution containing an inorganic or organic species contacts the modified anion exchange material in the presence of a DC current, free radical hydrogen and hydroxyl ion are produced at the semiconductor junctions, and the inorganic or organic species is reduced. If no species is present to be reduced, the free radical hydrogen decomposes to hydrogen ion.
In a third Sampson patent, U.S. Pat. No. 5,705,050 discloses that if the DC current is operated in a bipolar fashion, the oxidation or reduction efficiency is improved.
Beyond oxidation and reduction, water can be purified electrochemically by such processes as empty-cell and filled-cell electrodialysis. Electrodialysis is the process by which ions are removed by charge from an aqueous stream and is well known to those skilled in the art of electrochemistry. In electrodialysis, the ionic charge of the impurity causes it to migrate toward either the cathode or the anode. The electrodes are usually isolated from the water stream by ion exchange membranes so that as the ionic impurity is drawn toward an electrode with an opposite charge, it passes through a membrane of a similar opposite charge and into a waste stream, thus leaving the product stream purified.
In filled-cell electrodialysis, particulate ion exchange resin is placed between the ion exchange membranes to facilitate the transport of the undesirable ions through the membranes and into the waste streams. This resin, however, becomes partially contaminated, or exhausted, by the impurities in the water. Some regeneration of resin will occur when the resin between the membranes is a mixture of cation and anion resin, but the regeneration is limited. Where the particulate resin bed is beads, water is split, and hydrogen ion and hydroxyl ion are formed in the presence of DC current where the cation beads touch the anion beads, where the cation beads touch the anion exchange membrane, and where the anion beads touch the cation exchange membrane. The hydrogen ion then partially regenerates the cation beads, and the hydroxyl ion partially regenerates the anion beads. However, the production of the regenerant ions, hydrogen ion and hydroxyl ion, is limited, because there are relatively few sites where the cation beads touch the anion beads or the beads touch oppositely charged membranes. These sites where the oppositely-charged beads and membranes mechanically touch are called bipolar junctions.
It is generally known, however, that weakly ionized species, such as carbon dioxide and silica, are not easily removed by electrodialysis, because their ionic charges are too weak at a neutral pH to cause them to migrate toward the electrode and into the waste stream. Therefore, water containing weakly ionized species cannot reach the ultimate purity of 18 megohm-cm resistivity, because these weakly ionized species remain in the product water.
In addition, it is sometimes necessary to adjust the pH of water. Usually, pH adjustment is achieved by adding acid to lower the pH or caustic to raise the pH. In high purity water applications, however, the acid or caustic adds impurities to the water, which must then be removed.
Although empty-cell and filled-cell electrodialysis processes are well known and commonly used in water purification, no prior art process has demonstrated the capability to regenerate ion exchange resins isolated from the electrodes in order to effectively remove high concentrations of weakly ionized species, such as carbon dioxide and silica, or readily adjust the pH of the product water.
In describing the present invention, specific terminology will be resorted to for the sake of clarity. However, the invention is not intended to be limited to the specific terms so selected, or to the specific embodiments disclosed. It is to be understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar purpose, and the specific embodiments are intended to illustrate, but not limit, the broad technical application and utility of the present invention.
The present invention relates to an electrolytic reactor and process for electrolytically purifying water and adjusting pH in water solutions or streams. The reactor comprises an anode, a cathode and an ion exchange material provided between the anode and cathode. In the preferred form, the ion exchange material is particulate and the particulate ion exchange material forms a closely packed bed between the electrodes. As used herein, the term xe2x80x9cparticulate ion exchange materialxe2x80x9d includes granules, beads or grains of ion exchange material.
The ion exchange material can be in contact with either the anode or the cathode, or both, or separated from both the anode and cathode by suitable ion exchange membranes. The ion exchange material functions as an immobile electrolyte having an infinite number of transfer sites. The ion exchange material and its transfer sites facilitate ionic mobility between the electrodes when using a dilute solution containing a reactive ionic species.
The ion exchange material is treated or modified so that the electrolytic reactor will have numerous xe2x80x9csemiconductor junctionsxe2x80x9d incorporated into the ion exchange material. The semiconductor junctions are formed by permanently attaching an oppositely charged ionic species (counter ion) to the ion exchange material to occupy a percentage, preferably a minor percentage, of the exchange sites of the ion exchange material. Attachment of such a counter ion to an active site of the ion exchange material forms the semiconductor junction, which functions as an anode or a cathode, depending upon the ionic character of the counter ion, and acts as an electrocatalyst in the electrolytic reactions. The resulting modified ion exchange material is both ionically and electrically conductive.
While the ion exchange material of the electrolytic process and apparatus of the present invention is in many applications, a single species or monobed, such as all modified cation exchange resins or all modified anion exchange resins, it has been found that variation in the bed is possible in certain circumstances, and minor amounts of the opposite ion exchange material can be tolerated. In addition, while particulate ion exchange materials are the preferred physical form for the ion exchange material, other physical forms can be utilized. For example, ion exchange membranes and layered or lattice structured resin beds may have their ion exchange sites modified to form semiconductor junctions for use in the present invention.
Further, it has been found in accordance with the present invention that when a bed of modified ion exchange resin, whether a monobed of either cation exchange resin or anion exchange resin, or a mixed bed of both, is separated from both the anode and cathode, regenerant hydroxyl and regenerant hydrogen ions are formed at the semiconductor junctions. Typically, the resin bed is separated by ion exchange membranes. Ion exchange membranes suitable for isolating the electrodes from the modified ion exchange material are intended in their broadest context and include conventional ion selective membranes and diaphragms used in conventional electrodialysis, as well as all functional equivalents which segregate liquid and/or gas streams but allow selected ions to pass therethrough.
In accordance with this discovery, the ion exchange resins can be continuously regenerated electrically to displace the exchanged ions coming from a water stream being purified. Once these exchanged ions, or ionic impurities in the water, are displaced from the ion exchange resins, they leave the ion exchange chamber by conventional electrodialysis. The resulting purification of the water using the modified ion exchange materials in accordance with the present invention is much improved over conventional electrodialysis in that it has been found that weekly ionized species, such as carbon dioxide and silica, can be removed from the water stream to be purified. Further, it has been found, in accordance with the present invention, that the pH of the water can be adjusted, either up or down, in the water stream to be purified.
It is, therefore, an object of the present invention to provide an ion exchange material, modified to have semiconductor junction sites thereon, which can be employed in an electrolytic reactor to split water electrolytically and generate free radical hydroxyl, or free radical hydrogen, together with the respective regenerant hydrogen or regenerant hydroxyl ions in a suitable monobed, or both free radicals and regenerant ions in a suitable mixed bed.
A further object of the present invention is to utilize the modified ion exchange materials of the present invention in an electrolytic reactor for water purification or other treatment purposes, in which the semiconductor junction sites split water into regenerant hydrogen and/or hydroxyl ions which in turn electrolytically regenerate the resin material.
Another object of the present invention is to utilize the modified ion exchange material of the present invention in an electrolytic reactor to adjust the pH of the water stream.
These together with other objects and advantages which will become subsequently apparent reside in the details of the technology as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts throughout.